The present invention relates, in general, to solid state sensors, and more particularly, to solid state sensors for detecting contours in a traveling surface using reflected electromagnetic radiation.
Optical sensors, including combinations of light emitting diodes (LEDs) and photodiode detectors are used to monitor motion of moving objects. Currently available systems use a single LED aimed at a portion of a surface of an object being monitored, and usually use a single detector to detect a change in reflected light from the surface. The detector diode is positioned in the reflection path of the light reflected from the surface of the object being monitored. An example of such a system is shown in U.S. Pat. No. 4,850,712 issued to James B. Abshire on Jul. 25, 1989.
Since the single detector only responds to a change in reflectivity of the surface, the surface must be treated or marked to provide a clear change in reflectivity. For example, rotation of a wheel can be optically monitored by placing one or more "dark" or low reflectivity marks on a normally reflective surface of the wheel. As the wheel turns, the quantity of light reflected from the surface varies. This variation is then detected by the single photodiode in the reflection path.
One problem with current surface monitors is that it is that the reflectivity of common machine parts varies significantly as a result of surface roughness, cleanliness, and wear over time. Moreover, surface texture and finish must be carefully controlled for the surface monitor to work. This natural variation in reflectivity decreases the signal to noise ratio of the surface monitor, making it difficult to reliably detect the low reflectivity marks placed on the surface to be monitored.
What is needed is a surface monitor that is less sensitive to surface finish and texture, and can detect small changes in surface contour.